If you have the auto changeover regulator, most do, you aim it at one tank, but can open both. Then every day or so you check the indicator, if green, still on the same tank. If black, that tank it is empty and the regulator has switched tanks. Move the pointer to the the now "current" tank, remove the empty and have it filled, and replace. No real gauge to tell that I know of. Lot of "gimmicks", not no real way to tell short of removing the tank and weighing it. It has the weights stamped on the top ring.

The gauge one gives about 5 minutes from green to red even in BBQ use.
As stated, the tape on side one requires large propane use to show a reading as it uses a similar process as the water on tank technique.

I use the "manage till empty" technique Windrider uses.
Simple but effective and have always had heat at night.